1. Field of the Invention
This invention relates to lead-free projectiles, such as ballistic shot. More particularly, projectiles having a density approximating that of lead are formed by liquid phase sintering or liquid phase bonding.
2. Description of Related Art
Lead is the historic material of choice for projectiles such as bullets and ballistic shot. Lead is a very dense material having a room temperature density of 11.35 grams per cubic centimeter (g/cm.sup.3) where room temperature is nominally 20.degree. C. The high density enables lead-based projectiles to maintain a higher kinetic energy and more accurate flight pattern over long distances than less dense materials.
Lead is an environmentally undesirable material, particularly when the shot is fired over waterways and wetlands. A need exists for a projectile that is lead-free and environmentally acceptable.
One lead-free shot combines a material with a density greater than that of lead with a second, lower density, material in a proportion effective to achieve a density approximating that of lead. U.S. Pat. No. 5,399,187 by Mravic et al. discloses a sintered mix of powders having a high density component selected from the group tungsten, tungsten carbide and ferrotungsten and a more ductile, lower density component selected from the group tin, bismuth, zinc, iron, aluminum and copper. The powders are blended together, formed into a desired shape, compacted and sintered.
Solid phase sintering, as defined by the American Society for Metals, involves the bonding of adjacent surfaces in a mass of particles by molecular or atomic attraction on heating at high temperatures below the melting temperature of any constituent in the material. No matter how much compaction pressure is applied or how long the sintering time, it is difficult, when tungsten or ferrotungsten is a constituent of the powder blend, to achieve 100% of the theoretical density by sintering. A significant volume, on the order of 20% by volume, of the compacted mass is voids or porosity, thereby reducing the density of the sintered projectile.
One way to achieve 100% of the theoretical density is to form a homogeneous molten alloy of a higher density metal and a lower density metal. U.S. Pat. No. 5,264,022 to Haygarth et al. discloses a mixture of iron and 30%-45%, by weight, of tungsten, that is heated to a temperature of between 1650.degree. C. and 1700.degree. C. The molten alloy is then poured through a shot tower. While effective to generate a projectile having 100% of the theoretical density, the energy required to heat the tungsten/iron alloy to the melting point is prohibitive.
Another approach is to suspend the dense particulate, that typically has a very high melting temperature, in a molten bath of a lower melting temperature metal or metallic alloy. U.S. Pat. No. 4,881,465 to Hooper et al. discloses shot formed by suspending iron-ferrotungsten particulate, in a molten bath of a low melting temperature (under 300.degree. C.) lead-tin-antimony alloy. Approximately 25%-50%, by weight, of the mixture is the low melting alloy.
U.S. Pat. No. 5,189,252 to Huffman et al. discloses shot formed by suspending a dense particulate, such as tungsten or depleted uranium, in a liquid metal bath that is typically tin.
U.S. Pat. No. 5,279,787 to Oltrogge discloses shot formed by suspending a dense particulate, such as tungsten or tantalum, in a liquid metal bath that is tin, bismuth or an alloy such as bismuth-tin, bismuth-antimony, bismuth-zinc and tin-zinc. From about 11% to in excess of 60%, by weight, of the shot is the lower melting constituent. The Oltrogge patent discloses a counter-flow crucible for forming the molten suspension because the dense particulate settle from the molten bath and tend to form shot with an anisotropic density distribution. If the shot lacks uniform density, irregular shot patterns and unpredictable performance result.
It is also known to suspend a dense particulate, such as tungsten in a polymer matrix, such as polyethylene or a silicone rubber as disclosed in U.S. Pat. No. 4,949,645 to Hayward et al.
There exists, therefore, a need for lead-free shot and a method for the manufacture thereof that does not have the manufacturing problems of the prior art.